In severe pneumonia, dyspnea occurs due to hypoxia. Usually, in pneumonia, dyspnea is proportional to the extent of lung parenchymal lesion. When a patient with pneumonia presents with disproportionate dyspnea, other causes for dyspnea should be evaluated. Here, we present a 48-year-old female with pneumonia, found to have disproportionate dyspnea. Her hypoxia did not improve despite adequate antibiotic and oxygen therapy. On further enquiry, she admitted taking dapsone for leprosy. She had saturation gap diagnostic of methemoglobinemia. Diagnosis was confirmed by estimation of methemoglobin level in blood. She was treated with intravenous methylene blue and recovered. When a patient has disproportionate dyspnea, methemoglobinemia should be considered as a differential diagnosis.

A 48-year-old female homemaker was admitted with history of low-grade fever, generalized body ache, and weakness for the last 7 days. She also had developed dyspnea on exertion and dry cough for 3 days. She had no upper respiratory symptoms or other respiratory or cardiac symptoms. She had no history of premorbid lung or cardiac disease; no gastrointestinal symptoms; and no diabetes, hypertension, or systemic illness.

Question 1

Which of the following is the most likely diagnosis in this patient?

Atypical pneumonia

Upper respiratory tract infection

Cardiogenic pulmonary edema

Acute exacerbation of bronchial asthma

Lobar pneumonia.

Answer A

Atypical pneumonia.

Atypical pneumonia usually presents with low-grade fever, dry cough followed by dyspnea. She had no upper respiratory symptoms. She had no cardiac symptoms or cardiac disease in past. Dyspnea on exertion without any of the other cardiac symptoms, nocturnal dyspnea, and orthopnea exclude the possibility of cardiogenic pulmonary edema. She had no history of bronchial asthma and had no symptoms of bronchial asthma. Lobar pneumonia usually presents with high-grade fever, sometimes with rigor and chills with cough, expectoration and pleuritic chest pain.

Arterial blood gas (ABG) analysis and chest X-ray were done. Chest X-ray was normal. After ABG, the patient was started on 4 L of oxygen.

Question 4

In which of the following conditions, chest X-ray will always be abnormal in a patient with dyspnea?

Airway disease

Atypical pneumonia

Lobar pneumonia

Early interstitial lung disease

Pulmonary embolism.

In lobar pneumonia, chest X-ray will show consolidation. In all other conditions, chest X-ray may be normal at times.

Other causes for normal chest X-ray with dyspnea

Cardiac causes

Severe anemia

Subdiaphragmatic causes

Metabolic acidosis

Hemoglobinopathies.

ABG was taken with 4 L supplemental oxygen.

ABG: pH - 7.51; PCO2-15.2; PO2-162.8; HCO3-12.

Question 5

What is the diagnosis from ABG?

Answer

Respiratory alkalosis.

Question 6

Which of the following is least likely to cause respiratory alkalosis?

Pneumonia

Acute exacerbation of asthma

High-grade fever

Neurogenic pulmonary edema

Severe kyphoscoliosis.

Answer E

Severe kyphoscoliosis.

Severe kyphoscoliosis will lead to respiratory acidosis due to hypoventilation.

Other investigations

Hemoglobin - 10 g/dl

Total leukocyte count – 11,200 cells/cumm

Erythrocyte sedimentation rate - 75 mm/h

Platelet - 1.75 lakhs

Packed cell volume - 32%

Peripheral smear - mild normocytic, hypochromic anemia

Two-dimensional echo - normal.

Further story

The patient was treated with clarithromycin 500 mg twice daily and intravenous (IV) ceftriaxone 1 g thrice daily with oxygen 4 L. The patient improved clinically, but in spite of oxygen supplementation, 4 L SpO2 was 91%–92%. The degree of hypoxia was more than her clinical findings. PO2 in ABG was 106, not correlating with SpO2.

Question 7

What does pulse oximetry measures?

Hemoglobin level in blood

Amount of oxygen contained in blood

Pulse rate

Percentage of hemoglobin saturated with oxygen (SpO2)

SpO2 and heart rate.

Answer E

SpO2 and heart rate.

Normal SpO2 is 95%–99%.

Question 8

Which of the following does not interfere with pulse oximeter readings?

Dark skin

Nail polish

Hyperbilirubinemia

Dyshemoglobinemias

Hypotension.

Answer C

Hyperbilirubinemia.

Question 9

In which of the following poisoning/conditions, saturation gap is not seen?

Carbon monoxide

Methemoglobinemia

Cyan hemoglobin

Trinitrotoluene

Hydrogen sulfide.

Answer C

Saturation gap means disproportion in Pulse oximetry and ABG.

On further enquiry, she admitted taking dapsone since 6 months for Hansen's disease.

Question 10

What is the most likely diagnosis with this history of dapsone intake?

Co-oximetry is used to measure blood concentration of various forms of hemoglobin.

Normal range of methemoglobin in blood is <2%. Methemoglobin level above 70% is lethal. Severe symptoms with tissue hypoxia will occur when the level is above 20%. The color of the blood in methemoglobinemia is chocolate brown. Treatment for methemoglobinemia is methylene blue 1–2 mg/kg IV over 15 min. Methylene blue is contraindicated in glucose-6-phosphate dehydrogenase deficiency.[6],[7]

When breathlessness is out of proportion to clinical findings, atypical pneumonia, pulmonary vascular disease, other systemic causes, metabolic causes, or decreased oxygen carriage should be considered in differential diagnosis. Saturation gap is diagnostic of dyshemoglobinemias.